The poster depicts Prime Minister Vladimir Putin and reads: 'No!' The protests come three months before Mr. Putin—who was president from 2000 to 2008, and effectively remained the country's leader while he served as prime minister—is to seek a third term in office.

The poster depicts Prime Minister Vladimir Putin and reads: 'No!' The protests come three months before Mr. Putin—who was president from 2000 to 2008, and effectively remained the country's leader while he served as prime minister—is to seek a third term in office.

An aerial view of Bolotnaya Square in Moscow. In last Sunday's parliamentary vote, Mr. Putin's United Party narrowly retained a majority of seats, but lost its previously unassailable two-thirds share. Opponents say even that reduced performance was inflated by massive vote fraud.

An aerial view of Bolotnaya Square in Moscow. In last Sunday's parliamentary vote, Mr. Putin's United Party narrowly retained a majority of seats, but lost its previously unassailable two-thirds share. Opponents say even that reduced performance was inflated by massive vote fraud.